1. Field of the Invention
The invention relates to an ignition system for a spark-ignited internal combustion engine, and more particularly to a spark plug having high capacitance features.
2. Related Art
Ignition systems for spark-ignited internal combustion engines rely on a spark plug to produce a spark of sufficiently robust discharge so as to ignite a compressed air/fuel mixture. Often, more efficient ignition can be achieved by increasing the intensity of the spark.
The prior art has taught to incorporate a capacitor into the spark plug to increase the intensity of its spark. Various methods and configurations for integrating a capacitor into a spark plug have been proposed. All of the various proposed methods, however, have drawbacks and have failed to meet expectations in real world applications. Some designs integrating capacitors within the spark plug have failed to increase the spark intensity by any appreciable amount. Other designs are not capable of withstanding the high temperature, corrosive operating environment, and as a result their service life is limited. Still an additional limitation of spark plugs with integrated capacitors arises out of their mechanical fragility. These have been found not capable to withstand normal assembly operations without succumbing to chemical oxidation or destruction from collateral mechanical forces and abrasions.
One prior art attempt to achieve a higher capacitance spark plug suggested a metallic silver coating applied to the ID and OD of the alumina ceramic insulator, with the insulator forming an interposed dielectric. While this proposal has certain short term successes, it is subject to failure when used long term at high temperature. The failure mode is a high voltage dielectric failure of the ceramic due to deterioration of the ceramic resulting from migration of the silver into the alumina ceramic and reducing its effectiveness as an electrical insulator. Additionally, this prior design is highly susceptible to chemical oxidation, and the silver coating is not capable of withstanding subsequent assembly operations which include harsh, abrasive contact with machine tools and other elements.
Accordingly, there exists a need for a higher capacitance spark plug which is inexpensive to manufacture, conducive to existing spark plug manufacturing techniques and machinery, not subject to chemical oxidation or mechanical destruction during assembly operations, will not migrate into the matrix of the ceramic insulator, and which provides acceptable service life without deterioration or failure.